His rare press conference came amid mounting tension over reports that the Hague-based Special Tribunal for Lebanon was set to implicate the powerful Shia party, which is backed by Syria and Iran.
"There is a dangerous project that is targeting the resistance," Nasrallah added, using the standard Lebanese term for Hezbollah.
"We are not at all afraid, nor are we worried. We know how to defend ourselves."
He said that, from the outset, he had doubted the impartiality of the tribunal, which he accused of having made the decision to indict Hezbollah before conducting its probe.
"All the data we have indicate that the charge sheet was written up before even the interrogation of our members," he said. "The indictment is ready ... it is only a matter of political timing."
Al-Hariri and 22 others were killed in a massive bombing on the Beirut seafront on February 14, 2005.
The assassination sparked an international outcry and led to the withdrawal of Syrian troops from Lebanon after a 29-year presence.
The Hariri murder has been widely blamed on Syria but Damascus has consistently denied involvement.
'Lack of credibility'
The UN tribunal's president, Antonio Cassese, said earlier this year he expects an indictment in the case between September and December, sparking fears in already tense Lebanon of a repeat of the violence that brought the country close to a new civil war in 2008.
Nasrallah reiterated on Thursday accusations that the tribunal lacked credibility.
"As long as the probe does not look into the possibility that Israel is implicated, we believe it is biased," he said.
"Never has the investigation considered the hypothesis that Israel had the means and the motive" to assassinate Hariri.
Nasrallah, whose party fought a devastating 2006 war with Israel, in March confirmed the team investigating the Hariri murder had interrogated members of his party.
But he said at the time that he did not believe Hezbollah was in the tribunal's line of fire.